In a known power chain saw of this kind (cf. DE-28 16 485 and EP-O 347 869), a rear holding handle is provided on which the controls for switching the power chain saw on and off are disposed. In order to permit a change in the cutting plane of the sword, a rotatably mounted handle is articulated on the end face of the power saw. Compared with conventional handles, the rotatably mounted handle described in the two patent specifications displays a high standard from the safety engineering point of view.
Power chain saws are to be placed in the category of the most dangerous tools. The moving cutting chain can cause the severest injuries. The so-called "kickback" has proved to be the cause of most accidents. When "kickback" occurs, the cutting chain catches on the point of the guide bar, as a result of which the power chain saw is flung abruptly upwards and may be knocked out of the hand at the front being used to guide it. The hand at the rear, which continues to hold the power chain saw tightly, forms the point of rotation. The necessary averting reaction is scarcely possible, and the consequences are severe injuries to the arms, body and head.
As safety precautions, power chain saws conforming to the current state of the art are equipped with a hand-guard frame and a recoil mechanism, both of which can trigger the chain brake.
The chain brake is triggered in the event of a blow to the hand-guard frame, which is fitted to the upper side of the power chain saw. But if the hand is located on the holding frame at the side, this safety measure cannot come into operation. Even a hand-guard frame which rotates with the holding frame does not come into operation, since the point of rotation, in the event of the power chain saw being hocked upwards, is located at the rear holding handle.
In the case of power chain saws equipped with a recoil mechanism, the chain brake is triggered when the saw is knocked slightly upwards. The problem with the recoil mechanism is regulating the triggering value. If the setting is too light, the chain brake is constantly being triggered, and the rhythm of work is disrupted. Even if the power chain saw is knocked upwards in a plane other than that of the chain bar and is torn from the operator's hand, the moving cutting chain can still strike the operator's body. Because the saw is hocked upwards obliquely in relation to the plane of the chain bar, the triggering value of the recoil mechanism is not reached.
The way in which the chain brake incorporated in power chain saws works should be observed. The chain brake mechanism is linked to the hand-guard frame, which is located in a central position. The chain brake is released, but is already pre-tensioned. In the event of a forwardly directed blow to the hand-guard due to the power saw jerking abruptly upwards or any other reason, the chain brake stops the cutting tool. The lever of the chain brake is now located in the forward position. In order to release the chain brake, the hand-guard is pulled rearwardly (i.e. toward the rear handle) across the central position and into the rearward position. This causes the releasing of the chain brake (to allow the saw to be started), which, at the same time, pre-tensions the chain brake. When the hand-guard is released by the hand which pulled it into the rearward position, the hand-guard frame springs back automatically into the central position.